1. Field of the Invention
The present invention relates to ink jet printing and more particularly concerns an ink jet printer configuration which enhances ink droplet placement accuracy.
U.S. Pat. No. 3,596,275 to Sweet discloses a recording system wherein a sequence of ink droplets are directed to a recording medium in a controlled manner in order to encode that medium with information. Subsequent to the work done by Sweet, a variety of ink jet architectural designs have been proposed to enhance ink jet recording performance. These alternate designs have had as an aim, increased speed, improved resolution, reduced cost, and improved reliablity and maintainability.
A typical Sweet-type ink jet printer has one or more ink jet nozzles through which ink under pressure is directed toward a record medium which might, for example, comprise a sheet of paper. As ink is forced through the one or more nozzles, an exterior source of energy provides a perturbation to the ink to induce droplets of ink to break off at controlled intervals a well-defined distance from the ink jet generator. At the point of droplet breakoff, these droplets may be immediately charged by induction so that the droplet trajectory may be altered by a uniform electric field downstream from the droplet formation point.
The Sweet-type ink jet generators can be subclassified according to the particular configuration employed. In one type of arrangement, the ink droplets travel in a path dependent on their charge to a guttering system or in the alternative, the ink droplet is charged to avoid the guttering systems and travels to the paper. This architectural scheme is the basis for so-called binary ink jet systems. In the binary system, either a 1:1 correspondence exists between the number of ink jet nozzles and the incremental areas of coverage on the paper, or some type of relative transverse movement between generators and paper is provided so that one nozzle can throw ink to more than one picture element or pixel.
A second type of Sweet ink jet system employs a transverse scanning arrangement wherein once the droplets have been charged to an appropriate value, passage through the uniform electric field interposed between the generator and the record medium causes the ink droplet to scan transverse to the direction of paper motion. In this so-called "stitched" arrangement, a given ink jet nozzle supplies ink droplets to a number of incremental areas (pixels) on the paper. The term "stitched" derives from the fact that ink droplets from adjacent nozzles must be carefully positioned so they stitch together to completely cover the paper. It should be appreciated that for both a stitched type and binary type ink jet arrangement, relative longitudinal movement between the generator and the paper is provided as the ink droplets fly toward the paper.
One generic type ink jet printer uses a so called "drop on demand" drop printing technique. In this type system, relative movement between the paper and the ink jet generator is provided in a manner similar to the Sweet system. In the drop on demand system, however, ink droplets are generated only for those incremental areas on the paper where information is to be encoded. These systems require no guttering system since all droplets emitted from the generators strike the paper. A second feature of the drop on demand system is that no charging mechanism is required to alter the path of ink droplet travel. Each droplet follows a straight path to the paper so that no electric field generating apparatus is required. From the above it is apparent that both Sweet-type and "drop on demand" type jet printers have certain similarities, i.e. both configurations direct droplets of ink at a recond medium such as paper or the like, at controlled times to encode regions of the medium in a controlled way. The attraction of the "drop on demand" technique is that no charging and guttering equipment is required.
One perceived constraint on the "drop on demand" configuration is an upper boundary to the speed of information throughput such a system can handle. If, for example, the ink jet system is to be employed in a letter quality printer, it is presently believed a copy rate of about one page every thirty seconds is possible with the drop on demand system. While this speed may be adequate for a typewriter, it is not adequate for other ink jet applications. Those ink jet applications requiring high speed operation have favored the Sweet-type continuous drop production systems.
In a high speed ink jet copier/printer, the record medium must move past the ink jet generator at a fairly high rate of speed, and while doing so, each of the droplets generated must either be accurately directed to a particular paper position or to an ink gutter. Sources of inaccuracy of drop placement are encountered from either drop to drop electrostatic interactions or drop to air aerodynamic forces which divert the droplet from a preferred trajectory to the paper.
The aerodynamic interaction between a drop and the air in the vicinity of the drop would produce few, if any, adverse affects if the droplet were passing to the paper by itself without the slipstreaming effects caused by the presence of neighboring droplets close to a particular droplet. Each droplet would experience braking forces due to air resistance and deaccelerate uniformly. In a stream of droplets, however, those drops that lead the way experience greater braking than those drops in their wake. The lead drops spend a longer time in the deflecting field than does an identical droplet traveling in its wake. The increased time the droplet is deflected by the electric field causes a greater deflection of the drop and this difference in deflection caused by aerodynamic effects must be taken into account in a drop placement strategy.
The difference in drop speed caused by aerodynamic effects alters the placement strategy in a second way. It should be recalled that the paper is moving relative to the drop generator at a fairly high rate of speed. The braking cause by aerodynamic forces will cause an otherwise identically generated droplet to arrive at the paper plane later than a droplet traveling in the wake of a preceding drop. This difference in transit time again introduces a further source of drop misplacement.
The aerodynamic effects experienced by moving drops can also have affects on the drop to drop electrostatic interaction. Droplets experiencing greater aerodynamic braking will fall back into close proximity to faster moving drops. Since the drops are charged, this can result in either a merging together of two droplets or possibly an electrostatically generated bouncing away of one drop from another. Either phenomenon will disrupt the originally anticipated droplet trajectory and lead to drop placement error.
Electrostatic interactions in addition to the aerodynamically induced electrostatic interaction as mentioned above can affect the trajectory of the droplets in their travel to the paper plane. A first electrostatic interaction occurs as the droplets are being charged in a charging tunnel. Each of the three or four droplets preceding a given drop will induce a secondary charge on the drop as that drop is being formed. Unless compensated for at the time of droplet formation, this induced charge phenomena adds another source of droplet misplacement.
Even without the aerodynamic affect discussed previously, the electrostatic forces between drops in flight can deflect them from their intended trajectory and thereby cause droplet misplacement errors. Electrostatic interaction begins once the droplets are produced and continues until the droplet strikes either the paper or the gutter. Sweet-type architectures with a stitched drop configuration encounter particularly severe electrostatic interaction. In the stitched configuration, where bipolar scanning is used, i.e. droplets are both positively and negatively charged depending upon their desired trajectory, highly charged droplets directed to the gutter can have significant interactions with either negatively or positively charged droplets in close proximity to the gutter droplets. Droplets whose intended trajectory is to the paper can interact with the gutter ink droplets before deflection occurs. It is therefore seen to be desirable that the charge on all droplets be minimized so that electrostatic interactions are also reduced.
Once charged droplets enter the deflecting field, a drop may experience electrostatic attraction or repulsion as it begins to deflect away from the gutter trajectory. This phenomenon is particularly troublesome for those droplets in close proximity to highly charged gutter droplets in a bipolar system. The length of time a given drop spends close to a highly charged gutter drop varies inversely with the intended deflection of the droplet. A drop deflected to a pixel far away from the gutter stream experiences the least affect because of its rapid deflection away from the gutter stream. Conversely, drops directed to pixels in close proximity to the gutter stream experience the greatest electrostatic effects and therefore the most pronounced drop placement errors.
From the above it should be seen that so long as a charged droplet is moving through air in close proximity to other charged droplets, sources of drop placement inaccuracies are inevitable. It is an aim, however, of the present invention to reduce as much as possible, the deleterious effects such interactions cause.
2. Prior Art
Efforts to reduce the adverse affects caused by electrostatic and aerodynamic interactions between closely adjacent droplets are known in the art. U.S. Pat. No. 4,054,882, for example, discloses a technique for interlacing or non-sequentially directing ink droplets to a recording medium. The theory behind the technique disclosed in U.S. Pat. No. 4,054,882 is that once the droplets are charged, it is desirable that closely adjacent droplets be separated so that the inverse square drop off in coulomb interaction is experienced. An interlace strategy such as the one disclosed in U.S. Pat. No. 4,054,882 also reduces the aerodynamic interactions between closely adjacent droplets in the droplet stream. A more uniform aerodynamic breaking effect is experienced by each of the droplets in the stream rather than some droplets having their path shielded by previous droplets in the sequence.
Another technique known in the art for reducing electrostatic and aerodynamic interactions is the use of guard drops. Guard drops are drops which are directed to the gutter but separate those droplets which are intended to strike the paper. Use of guard drops is inefficient since all guard drops are guttered and never used for printing.
While U.S. Pat. No. 4,054,882 addresses the aerodynamic and electrostatic interaction between droplets, practice of the present invention further reduces the adverse effects of these phenomenon and in particular reduce these effects in a bipolar scanning type Sweet system. It should be appreciated that bipolar scanning systems are not new per se, but that the present invention relates specifically to an improved bipolar system in which the interaction between droplets and air are reduced. U.S. Pat. No. 3,877,036 to Loeffler et al., for example, discloses a bipolar scanning configuration wherein both positively and negatively charged droplets are directed to an electric field which causes those droplets to impinge upon a record medium at a location dependent upon the magnitude of the charge. While both bipolar and interlace strategies exist in the prior art, to applicant's knowledge, there has been no suggestion to modify the conventional bipolar and/or interlace strategy in conformity with the technique disclosed in the present application.